Blood Draws and Lab Tests: What to Expect

Blood tests — also called labs or laboratory studies — are a routine and essential part of IV therapy and vascular access care. They help your care team verify that your treatment is working, ensure medications are at safe concentrations, monitor your organ function, and detect problems early.

This guide explains why blood tests are needed, how blood is collected (including through your IV catheter), what to expect, and how to manage common concerns like difficult veins or needle anxiety.

Why Blood Tests Are Needed During IV Therapy

To monitor your medication levels

Some IV medications — particularly antibiotics such as vancomycin, gentamicin, and tobramycin — must be maintained within a specific concentration range in your blood. Too little and the drug may not be effective against the infection; too much and it may damage your kidneys or cause other toxicity. Blood is drawn at scheduled times to measure these levels and allow dose adjustments.

To monitor organ function

Many IV medications affect the kidneys, liver, or other organs. Regular blood tests check:

Creatinine and BUN — kidney function
Liver enzymes (ALT, AST, bilirubin) — liver function
Electrolytes (sodium, potassium, magnesium, phosphorus) — balance of important salts your body needs

To monitor blood cell counts

Complete blood count (CBC) monitors:

Red blood cells (and hemoglobin/hematocrit) — to detect anemia
White blood cells — elevated counts may indicate infection or inflammation; very low counts (in patients on chemotherapy) indicate infection risk
Platelets — important for clotting; some medications affect platelet counts

To monitor infection response

Blood markers such as CRP (C-reactive protein), ESR (erythrocyte sedimentation rate), and white blood cell counts can show whether an infection is resolving in response to treatment.

To draw blood cultures

If infection or fever occurs, blood is drawn specifically to grow any bacteria or fungi in a laboratory — this is called a blood culture. The results identify the specific organism causing infection and which antibiotics will kill it. Blood cultures usually require 2–5 days for results.

How Blood Is Drawn: Two Main Methods

1. Venipuncture (standard blood draw)

A phlebotomist, nurse, or medical assistant inserts a small needle directly into a vein — usually in the antecubital fossa (the inside of the elbow), the forearm, or the hand — draws the required amount of blood into one or more collection tubes, and then removes the needle. The whole process takes about 1–3 minutes for straightforward draws.

This is the standard method when you do not have a central catheter, or when certain tests are more accurate from a fresh vein stick rather than a catheter.

2. Drawing blood from your catheter

When you have a PICC line, central venous catheter, tunneled catheter, or implanted port, blood can often be drawn directly through the catheter — eliminating the need for repeated arm sticks.

Advantages: Convenient, no additional needle insertion (for PICC/CVC), less discomfort over a long treatment course, avoids wear on peripheral veins.

Important considerations:

Blood drawn through a catheter that is currently running an infusion requires the infusion to be paused and a “discard” amount of blood to be withdrawn first (to flush out any diluted blood or residue from the catheter lumen) before the actual sample is collected. Your nurse manages this process.
Some laboratory tests are less accurate when drawn through certain catheters, or may require specific tube types or timing. Your care team will advise when catheter draw is appropriate and when a separate venipuncture is needed.
PICC lines and midlines: Blood is routinely drawn through PICCs. Some single-lumen PICCs may not draw blood easily — your nurse will troubleshoot this.
Implanted ports: Drawing blood from a port requires accessing it first with a Huber needle (exactly as for infusion). This may be done as part of a scheduled infusion visit, or specifically for lab draws.
Peripheral IVs: Peripheral IVs are generally not used to draw blood after the initial placement (blood drawn through them can be inaccurate, and repeated aspiration can damage the small veins and the IV itself). Fresh venipuncture is preferred.

The Blood Draw Process: Step by Step

Standard venipuncture

The phlebotomist or nurse selects a suitable vein — usually the antecubital area or forearm. A tourniquet is applied briefly above the site to help the vein become more visible and accessible.
The skin is cleaned with an alcohol wipe and allowed to dry.
The needle is inserted. You feel a brief, sharp sting.
Blood is drawn into collection tubes. You may feel a dull pulling sensation.
The tourniquet is released, the needle is removed, and gentle pressure is applied for 1–2 minutes.
A small bandage is placed.

Drawing from a PICC or central catheter

The nurse verifies the catheter is correctly positioned and flushing freely with saline.
If an infusion is running, it is paused.
A discard volume of blood is withdrawn and discarded (to remove catheter residue and diluted blood).
The required blood samples are drawn directly through the catheter.
The catheter is flushed thoroughly with saline after the draw (and with heparin if prescribed) to clear any residual blood and prevent clotting.

Lab Schedules: What to Expect

During IV antibiotic therapy, a typical monitoring schedule might look like:

Weekly: Complete blood count, creatinine, liver function, electrolytes
Twice weekly (or more): Drug levels for some medications (e.g., vancomycin trough levels)
As ordered: Blood cultures if fever develops

Your care team will give you a specific schedule. Labs can be drawn by:

Your home infusion nurse during a home visit
A mobile phlebotomy service that comes to your home
A local outpatient lab near your home
At your follow-up appointments
At an infusion center if you are attending for treatment

Plan ahead: Know where your labs are to be drawn and ensure appointments are scheduled in advance, particularly for time-sensitive tests (like vancomycin trough levels, which must be drawn at a specific time relative to the dose).

Understanding Common Lab Tests in Vascular Access Care

Vancomycin levels (AUC monitoring)

Vancomycin is a powerful IV antibiotic used for serious infections. Its levels are measured using a method called AUC (area under the curve) monitoring, which requires drawing blood at specific time points. Your pharmacist will tell you exactly when the blood needs to be drawn.

Normal result: Individualized by patient weight, kidney function, and infection type — your team sets your target.

Creatinine (kidney function)

Creatinine is a waste product filtered by the kidneys. An elevated creatinine suggests the kidneys may be under stress — important information when receiving antibiotics that can be nephrotoxic (kidney-affecting).

Electrolytes (sodium, potassium, magnesium, phosphorus)

Levels outside the normal range can cause symptoms including muscle cramps, weakness, irregular heartbeat, and confusion. IV fluid therapy and some medications affect these levels. Supplements are often given IV or orally to correct deficiencies.

Complete Blood Count (CBC)

Hemoglobin: Oxygen-carrying protein in red blood cells. Low hemoglobin = anemia. You may feel tired, pale, or short of breath.
WBC (white blood cell count): Elevated = infection/inflammation response; very low = increased infection risk.
Platelets: Needed for clotting. Very low platelets = bleeding risk.

C-Reactive Protein (CRP)

A marker of inflammation and infection in the body. Rising CRP may indicate worsening infection; falling CRP suggests the infection is responding to treatment.

Blood cultures

If your blood is positive for bacteria (or fungi), the lab identifies the organism and tests it against multiple antibiotics to determine which ones it is sensitive to (this is called sensitivity testing or antibiogram). This helps your physician prescribe the most targeted and effective antibiotic.

Difficult Veins: What to Know and Do

Some patients have small, deep, rolling, or fragile veins that are hard to access, resulting in multiple needle attempts. This is frustrating but common. If you have a history of difficult veins:

Tell your care team in advance. Knowing this allows them to allocate extra time, use a more experienced phlebotomist, or plan for ultrasound-guided blood draw when needed.

Ask for an experienced phlebotomist or nurse. It is entirely appropriate to say: “I have very difficult veins and usually need a very experienced person to draw my blood. Is there someone with extra experience available?”

Ask about ultrasound guidance. Ultrasound can visualize veins that are invisible on the surface. Some facilities offer ultrasound-guided blood draws for patients with challenging access.

Request warmth. Warm veins are easier to access. Before a draw, apply a warm blanket or warm compress to your arm for 5–10 minutes, or stay warm. Cold causes veins to constrict.

Stay hydrated. Dehydration causes veins to shrink. If you know a blood draw is coming, drink water in the hours before (unless instructed not to).

Advocate for your catheter. If you have a PICC or central catheter and are being asked to undergo repeated arm sticks, ask your nurse: “Can this blood be drawn from my catheter?”

Request a limit on attempts. It is reasonable to tell your phlebotomist: “I find multiple attempts very distressing. If you are not successful after two tries, I would like someone else to try or for us to use a different approach.” Most healthcare facilities have policies supporting this.

Needle Anxiety and Fear of Blood Draws

Fear of needles (trypanophobia) and anxiety about blood draws are extremely common — affecting up to 25% of adults. There is no need to feel embarrassed about this. Your care team wants to know so they can help make the experience as manageable as possible.

Strategies that help many patients

Tell your team. “I have significant anxiety about needles” is a valid and important piece of information. Simply sharing this often results in more patience, gentler technique, extra reassurance, and additional time.

Topical numbing cream. EMLA cream or LMX4 cream, applied to the skin 45–60 minutes before a blood draw, can significantly reduce or eliminate the sensation of the needle. Ask if this can be arranged for you.

Distraction. Looking away, focusing on a fixed point on the wall, listening to music through headphones, talking about something unrelated, or using relaxation apps can reduce anxiety during the procedure.

Controlled breathing. Slow, deep breaths — breathing in for 4 counts, holding for 2, out for 4 — activate the parasympathetic nervous system and reduce the physical stress response. Practice before the draw.

Ask for a slow approach. Phlebotomists can insert needles slowly and gently. Asking for this is reasonable.

Lie down. Fainting (vasovagal syncope) during blood draws is common in people with needle anxiety. If you have fainted or felt faint before, ask to lie down for the draw. This significantly reduces the risk of fainting. Tell your phlebotomist that you have a history of near-fainting; they will watch for signs and have you positioned safely.

Applied tension technique (for fainting-prone patients): Research shows that tensing the muscles in your legs and abdomen during a blood draw can prevent fainting by maintaining blood pressure. Your care team can teach you this technique.

Topical cold. A brief spray of a cooling agent over the skin can numb it momentarily — ask your nurse if this is available.

For severe phobia: If needle anxiety significantly impairs your ability to receive medical care, ask your physician for a referral to a psychologist or therapist who specializes in medical phobias. Evidence-based treatments (including cognitive-behavioral therapy and exposure therapy) are highly effective and can make a substantial difference in your quality of care over time.

After the Blood Draw: What’s Normal

A small bruise at the draw site is common, especially if multiple attempts were needed or the vein was fragile. It will resolve over 5–10 days.
Mild tenderness at the draw site for a day or two is normal.
Apply gentle pressure at the site for at least 1–2 minutes after needle removal to minimize bruising.
If bruising is significant or spreading, or the site is very tender and swollen, apply a cold pack and notify your care team.

When Lab Results Need Your Attention

Your care team will communicate results that require action. You do not need to chase down every result, but it is reasonable to ask:

“Have my lab results come back?”
“Are all my values within the expected range for my treatment?”
“Does anything need to be adjusted based on my labs?”
“When are my next labs scheduled?”

If you have access to a patient portal, you may see results posted before your care team has called to discuss them. Lab values flagged as “high” or “low” are not always clinically significant — the clinical context matters. Wait to hear from your care team for interpretation, or call if you are concerned.

This guide is for educational purposes and is not a substitute for advice from your own healthcare team. Always follow the specific instructions given to you by your nurses, doctors, and laboratory staff.